Steam turbines are generally designed to operate at relatively uniform cover-to-base temperatures and the clearances within such turbines for various seals and bearings are established for operation of the turbine at such uniform temperatures. Although the turbines experience variable temperatures during start-ups and shut-downs, so long as the temperature over the extent of the turbine is maintained at a relatively uniform cover-to-base values, the turbine can accommodate such temperature variations. However, if the temperature between the upper half of a turbine varies significantly from temperatures within the lower half of the turbine at the same axial location, distortion of the turbine may occur and result in damage due to contact between rotating components within the turbine system. Such temperature variations are generally more predominate during turbine shut-down when active steam flow has been terminated into the turbine. In a shut-down mode, the turbine experiences a cooling of the casing and other metal parts. Normally, the turbine rotor is maintained in rotation at a relatively low speed using an external drive in order to prevent sagging or deformation of the rotor if it were allowed to come to a full rest. The casing surrounding the turbine may be subjected to differential temperatures for various reasons including simply that the higher temperature residual steam-air mixture within the turbine at shut-down may rise to the top of the turbine casing while the cooler steam-air mixture may settle to the bottom. In addition, various steam extraction lines connected to the turbine may experience a reverse flow as the pressure relationships among the turbine casings, piping systems and connected pressure vessels change after steam flow has been terminated, and cooling of the various components takes place at different rates. The cooling of the varying subatmospheric pressures not encountered during active turbine operation may cause the steam or water mixtures within the extraction lines to back-up into the turbine casing and increase the rate of cooling of the casing. The major effect of such cooling may cause the upper portion of the casing, hereinafter referred to as the cover, to be at a different, usually higher temperature than the lower portion of the casing, hereinafter referred to as the base. Since the turbine casing is essentially an elongated housing, the effect of a temperature differential between the cover and the base is to cause the casing to attempt to bend or arc. Such casing deformation may result in contact between rotating components on the rotor and various elements attached to the casing. Any such contact may contribute to internal damage requiring maintenance before the turbine can be restarted.
U.S. Pat. No. 4,584,836 describes one solution to the problem of unequal cooling of turbine casings. As described therein, one solution to controlling the temperature variation of the turbine casing is to cover the casing with what is essentially an electric blanket. The disclosed system monitors the temperature of the casing and adjusts power applied to various sections of the electric blanket covering the casing in order to maintain essentially uniform temperature of the casing over the extent of the turbine. While the use of the electric blanket is satisfactory for maintaining turbine casing temperature, this particular solution is relatively expensive and, in addition, requires that a blanket having electrical wiring be positioned about a thermally conductive surface. Furthermore, this system requires electric power in order to operate the electric blankets and as such consumes relatively large amounts of energy. Accordingly, it would be desirable to provide a method and apparatus for maintaining casing temperature equilibrium without the use of expensive, specially designed electric blankets.